1. Field of the Invention
The present invention relates to a solid-state imaging device providing high image quality and low power consumption and, especially, a solid-state imaging device, having a ring-shaped gate, which can obtain signal outputs by modulating threshold voltages (Vth) in accordance with subject light.
2. Related Art
As solid-state imaging devices to be built in cellular phones, etc., there are two types of image sensors: charge-coupled device (CCD) image sensors; and CMOS image sensors. CCD image sensors are superior in image quality, while CMOS image sensors consume lower power and process cost. In recent years, proposals have been made for MOS imaging devices employing threshold voltage modulation which provides both high image quality and low power consumption. A type of MOS imaging device providing threshold voltage modulation is disclosed in, for example, Japanese Patent Serial No. 2513981.
The image sensor disclosed in Japanese Patent Serial No. 2513981 obtains image outputs by arranging sensor cells, which correspond to unit pixels, in a matrix and repeating a three-state cycle of initialization, storage and read-out. Each unit pixel of the image sensor has a photodiode for storage, a modulation transistor for read-out, and an overflow drain gate for initialization. The gate of the modulation transistor is ring-shaped.
A light-generated charge generated by incident light entering the photodiode is transferred to a P-well region provided under the ring gate and stored in a carrier pocket formed in the region. The light-generated charge stored in the carrier pocket changes the threshold voltage (Vth) of the modulation transistor, which makes it possible to obtain a pixel signal corresponding to the incident light from a terminal (source contact) coupled to the source region of the modulation transistor.
FIG. 8 is a schematic top-view drawing of a modulation transistor unit of an image sensor.
The image sensor shown in FIG. 8 has a photodiode 110 and a modulation transistor 101 adjacent to each other for each unit pixel on a board. A gate 102 of the modulation transistor 101 is formed in a ring. In an opening at the center of the ring gate 102, a source region 104 is formed. A gate contact is formed, though not illustrated, on part of the surface of the ring gate 102. Provided around the ring gate 102 is a drain region 106. A light-generated charge (carrier) generated by incident light entering the photodiode 110 is stored in a carrier pocket 108, which is a ring-shaped narrow band formed in a P-well region provided under the ring gate 102. When the stored charge changes the threshold voltage (Vth) of the modulation transistor 101, a pixel signal corresponding to the incident light can be output from a source contact 105 coupled to the source region 104 of the modulation transistor 101. By applying bias voltages of, for example, 5V and 0V to a drain contact 107 and the source contact 105, respectively, electric current flows between the drain and the source in accordance with the intensity of incident light entering the photodiode 110, and a signal output is output from the source contact 105.
If a locally low-potential region 109 is formed at part of the carrier pocket 108 under the ring gate 102 due to, for example, an uneven impurity concentration within the well, carriers (holes) are not stored uniformly in the carrier pocket 108 which is ring-shaped, but stored first in the locally low-potential region 109 when light enters the photodiode 110. If there is no locally low-potential region 109, an electric current-flow path (hereinafter referred to as “current path”) from all directions of the region (drain region 106) outside the circumference of the ring gate 102 toward the source region 104 is formed, and electric current flows uniformly on this path. In the latter case, a signal starts being output linearly from the time the amount of stored electric charges is still small (see line g in FIG. 9). Whereas, if there is the locally low-potential region 109, electric current does not flow from all directions of the region (drain region 106) outside the circumference of the ring gate 102 toward the source region 104, but starts flowing into the locally low-potential region 109. Therefore, low light intensity results in the slow rising of signal outputs, worsening linearity (see line h in FIG. 9), and thereby causing dark defects.
Similar problems are disclosed in, for example, FIG. 15 and FIG. 17 of Japanese Unexamined Patent Publication No. 10-65138. However, in Japanese Unexamined Patent Publication No. 10-65138, a countermeasure is taken by forming a current path in a specific region in accordance with impurity distribution instead of a configuration forming a gate in a ring shape. Also, examples of forming a ring-shaped gate are shown in Japanese Unexamined Patent Publication No. 2002-164527, as well as Japanese Patent Serial No. 2513981 (discussed above).
As described above, local non-uniformity of potential in the carrier pocket on the ring gate has been causing a problem of dark defects when the amount of stored electric charges is small, that is, when the light intensity is low.
Accordingly, the present invention has been developed in view of the above problem and aims to provide a solid-state imaging device which has preferable linearity of signal outputs according to light intensities and does not cause dark defects even at a low light intensity.